Calorie (th) per Minute per Square Centimeter [cal(th)/(min·cm²)]

The unit calorie (th) per minute per square centimeter, written as cal(th)/(min·cm²), measures heat flux density or thermal power per unit area using the thermochemical calorie (1 cal(th) = 4.184 J). It represents the amount of heat energy transferred through 1 cm² of surface every minute.

This unit is often used in laboratory experiments, solar energy studies, and material testing, particularly when small surfaces and longer exposure times are involved. Applications include:

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  Solar radiation measurements on small panels or surfaces

  
  
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  Thermal testing of materials over extended periods

  
  
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  Calorimetry experiments on small samples

  
  

Mathematically, the heat flux is expressed as:

where q is heat flux in cal(th)/(min·cm²), Q is energy in calories, A is area in cm², and t is time in minutes.

Conversion to SI units:

Although SI units such as W/m² are standard, cal(th)/(min·cm²) remains useful in historical calorimetry, laboratory experiments, and solar radiation studies, providing an intuitive measure of energy transfer per minute per small surface area.

Metric Horsepower per Square Foot [hp(metric)/ft²]

The unit metric horsepower per square foot, written as hp(metric)/ft², measures power or energy flux density per unit area using metric horsepower. One metric horsepower is defined as 735.5 watts, slightly less than the mechanical (Imperial) horsepower of 745.7 W. Dividing by square feet gives the rate of power delivery or absorption per unit area.

This unit is used in engineering, industrial design, and energy flux studies where power distribution per unit surface area is analyzed. Applications include:

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  Mechanical equipment power output per platform area

  
  
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  Energy flux in industrial processes

  
  
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  Assessment of high-power machinery energy delivery

  
  

Mathematically, the power flux can be expressed as:

where q is power per unit area (hp(metric)/ft²), P is power in metric horsepower, and A is area in ft².

Conversion to SI units:

While W/m² is the SI standard, hp(metric)/ft² is convenient in regions using metric horsepower, especially in European engineering and historical industrial literature, providing an intuitive measure of high power density per unit surface area.